It is known to provide a shaver or razor that relies on a laser for cutting hair rather than an arrangement of cutting blades. Shavers without blades have fewer moving parts and so wear is reduced, which provides an advantage over mechanical shavers. Furthermore, the use of a laser can reduce skin irritation as there are no sharp objects that contact the skin surface. Laser shavers work by optical absorption, also known as laser ablation, in which hair exposed to a laser beam absorbs the energy of the beam, causing it to be vaporised and/or severed.
Shaving performance is typically measured by two criteria closeness of shave and irritation of the skin. Therefore, a good performing shaver should minimise the remaining hair length by positioning the laser as close as possible to the skin. However, this may cause more skin irritation if heat and energy from the laser is incident on the skin. It is necessary to protect the skin from contact with the laser beam to avoid damaging or irritating the skin being shaved. Hair trimmers or groomers are used to trim hair to a constant length, so although closeness is not a major performance factor, uniformity of remaining hair length is desirable.
It is known, for example from WO 95/33600, to generate a laser beam that is positioned parallel to the skin and perpendicular to the stroke direction to cut hairs as the shaver is moved over the skin. However, Gaussian theory dictates that laser beams have a natural intensity variation along their length. Beams will have a focal point where the laser beam has maximum intensity (power per unit area) and minimum width, meaning the focal point is the most effective part of the laser beam for severing hair by optical absorption. On the other hand, parts of the laser beam furthest from the focal point will have a larger width and therefore a more distributed intensity and will not be as effective at severing hair because the energy of the laser beam is incident over a larger area of hair. Therefore, there is a variation in hair severing performance along the optical axis of a laser beam and this may result in uneven hair severing and non-uniform closeness. Moreover, the power density distribution in each position over a cutting zone in which hairs are received is not always sufficient to cut hair. Due to the natural divergence of Gaussian laser beams it is impossible to achieve a uniform beam thickness along an optical axis.